# in code which calls 'who':
  $_[HEAP]->{reason_for_calling_who} = 'whatever';
  $irc->yield('who');
  ...

# in on_who handler:
  if ($_[HEAP]->{reason_for_calling_who} eq 'whatever') {
    # complete processing for this case
  } elsif ($_[HEAP]->{reason_for_calling_who} eq ...) {
    ...
  }